It has been previously shown that reduced insulin A and B chains can be reoxidized together to give the native hormone with a reasonably good yield. Later, the reduction and oxidation of proinsulin was found to give the prohormone with a yield only slightly better than that for the resynthesis of the active hormone from its chains. It seems probable that the insulin A and B chains already contain enough information in their sequence so that the chains are more or less correctly paired before oxidation to form the covalently linked structure. The present study will investigate the pairing of the insulin A and B chains in solution through the interaction of their side chain groups by physico-chemical methods. It is also proposed to define the mode of interaction and the contribution of the side chain groups by selective hybrid recombination of the chains from different species and by chemical modification of these side chain groups. From the knowledge thus obtained, it would be possible to increase still further the yield of insulin by recombination of the chains which is of practical importance. Protein-protein interactions through their side chains are of great significance in life's processes. A detailed knowledge at the molecular level of the interaction of the insulin chains would serve not only as a starting point for the study of these interactions in general, but also throw some light on the folding of the molecule during its synthesis. Moreover, during the course of the present study, a large number of differently modified insulins and hybrid insulins will be made available. The possibility that the wide spectrum of the biological effects of insulin might involve different receptors could be studied with insulin analogs modified at different part of the molecule and the possible clinical significance explored.